The Midcourse Space Experiment (MSX) spacecraft has now been in orbit for over 8 years. The Total Pressure Sensor (TPS) has continually made measurements of the ambient and spacecraft induced environment since launch on April 24, 1996. The environment surrounding the satellite continues to be very dynamic. The magnitude of the solar induced pressure peaks has not decayed with time as would be expected. Even sole consideration of measured pressure versus duration of solar illumination does not illustrate long term decay. The TPS has continued to measure pressure variations of more than 2 orders of magnitude as the sun illuminates the blankets surrounding the instrument. The multi-layer thermal blankets appear to represent a large virtual source of water. The measured pressure is highly dependent upon the solar angle and the shadowing of surfaces by other structures. We will focus our attention on the results from an identical set of experiments conducted each year in July.
KEYWORDS: Space operations, Contamination, Satellites, Databases, Optical properties, Cryogenics, Data modeling, Reflectivity, Atmospheric particles, Mars
This paper provides a review and discussion of current and future efforts associated with the NASA’s Space Environments and Effects (SEE) Program. An update of the Satellite Contamination and Materials Outgassing Knowledgebase is also presented. Recent additions to the Knowledgebase are discussed.
This paper describes the development of a facility for material outgassing measurements using quartz crystal microbalances (QCMs) operating at temperatures as low as 20K. The objective of this effort was to develop a system that operates in the 4 to 30K temperature range and that provides material outgassing data at much lower temperatures than have previously been available. The desired measurements are based on the ASTM Standard E1559 test method. Many space-based infrared sensor systems operate at temperatures much colder (i.e., 4 to 30K) than the 77K temperature commonly used in the E 1559 test method. The data collected will be used to compare material outgassing data collected at 77 and 20K to determine the differences in total mass loss (TML) measured at the two temperatures. This will provide an answer to the question that for a long time has been associated with the E1559 test method: Does the 77K QCM collect essentially all of a material's outgassed products??.
Molecular surface contaminants can cause degradation of optical systems, especially if the contaminants exhibit strong absorption bands in the region of interest. Different strategies for estimation of spectral degradation responses due to uniform films for various types of systems are reviewed. One tool for calculating the effects of contaminant film thickness on signal degradation in the mid IR region is the simulation program CALCRT. The CALCRT database will be reviewed to correlate spectral n and k values associated with specific classes of organic functional groups. Various schemes are also investigated to estimate the spectral degradation in the UV-Vis region. Experimental measurements of reflectance changes in the IR to UV-Vis regions due to specific contaminants will be compared. Approaches for estimating changes in thermal emissivity and solar absorptivity will also be discussed.
A laboratory prototype reflectometer with applications in scene content characterization is available at the Arnold Engineering Development Center (AEDC), Arnold AFB. The SCAT/R measures specular, total, and retro-reflectance at five angles of incidence (AOI) scanning from 2.5 to 15 micrometers . Diffuse reflectance, total integrated scatter (TIS), and thermal emittance are calculated from the measurements. Conventional TIS measurements provide a good measure of a surface finish deviation from specularity, but the scatter distribution is not obtainable from a single measurement. By making TIS measurements over a series of AOI, information on the distribution is obtained. For example, a diffuse surface can be evaluated to determine whether the scattering is characterized by a Lambertian distribution. It has been observed that a series of retro-reflectance measurements at various AOI yields similar angle resolved information. In this paper, the relationship between retro-reflectance and TIS as a function of AOI is investigated for various materials using data from SCAT/R scans. The presentation and analysis of the data follow a brief description of the instrument. The SCAT/R thermal infrared data is useful in identifying/cataloging polarization and hyperspectral characteristics of materials and coatings used for camouflage and for other target and background applications.
The Midcourse Space Experiment (MSX) spacecraft was specifically designed and processed to minimize contamination. This spacecraft represents a best case scenario of spacecraft induced environment. The contamination instrument suite consisted of 10 sensors for monitoring the gaseous and particulate environment. The Total Pressure Sensor (TPS) has continuously measured the ambient local pressure surrounding MSX since its launch on April 24, 1996. The sensor's primary goal was to monitor the early mission (less than one week) ambient pressure surrounding the spacecraft's optical telescopes and to indicate when environmental conditions were acceptable for opening the protective covers. However, the instrument has illustrated that it is quite robust and has successfully measured the long-term decay of the pressure environment. The primary constituent of the atmosphere is water outgassed from the thermal blankets of the spacecraft. The water-induced environment was expected to rapidly decay over the first few months to levels more closely approaching the natural environment. The data generally shows decay toward this level, however, the pressure is quite variable with time and can be influenced by discrete illumination and spacecraft orbital events. Several experiments conducted yearly indicate that the thermal blankets retain significant quantities of water. The local pressure due to water vapor is shown to increase by a factor of 100 from direct solar illumination. Moreover, the multi-layer construction of the blankets causes them to form a deep reservoir that continues to be a source of water vapor 3+ years into the mission. We will present pressure data from several experiments, each separated by one orbital year, that exhibit these water vapor induced pressure busts. The decay and longevity of these bursts will also be discussed.
The Midcourse Space Experiment (MSX) satellite was launched on April 24, 1996. This paper provides an update of the quartz crystal microbalance (QCM) data accumulated over these last four years in space. The MSX is the only known experiment that has provided continuous contamination monitoring for such an extended length of time. The five QCMs on board the satellite have provided on-orbit data that have been invaluable in characterizing contamination levels around the spacecraft and inside the cryogenic Spatial Infrared Imaging Telescope (SPIRIT 3). One of the QCMs, the cryogenic QCM (CQCM), located internal to SPIRIT 3, was mounted adjacent to the primary mirror and provided contamination accretion measurements during the 10-month lifetime of SPIRIT 3. Real- time monitoring of contaminant mass deposition on the primary mirror was provided by this CQCM which was cooled to the same temperature as the mirror - approximately 20K. Thermogravimetric analyses (TGAs) on the CQCM provided insight into the amount and species of contaminants condensed on the SPIRIT 3 primary mirror during various spacecraft activities. The four temperature-controlled QCMs (TQCMs) were mounted on external surfaces of the spacecraft for monitoring spacecraft contamination deposition. The TQCMs operated at approximately -50$DEGC and were positioned strategically to monitor the silicone and organic contaminant flux arriving at specific locations. Updated time histories of contaminant thickness deposition for each of the QCMs are presented. Gradual contaminant thickness increase was observed during the first year in space. During the second year, the QCM frequencies (contaminant film thickness) began to decrease, with the time of onset depending on QCM location. Possible explanationsfor this interesting behavior are discussed.
We present a summary of the particle environment surrounding the Midcourse Space Experiment (MSX) satellite after 32 months on orbit, including two discrete particle releases produced by micrometeoroid or debris impact. We report on the characteristics of that environment, including particle occurrence rates, velocities, size distributions and trends in the environment. To our knowledge, the long term particle contamination observations that we have made on MSX are the first of their kind. The particle occurrence rate decreased steadily during the first year on orbit, but then remained at a constant level after 32 months on orbit. Our estimate of the total number of particles on the spacecraft surfaces at launch. We conclude that environmental effects such as UV, radiation, thermal cycling, and micrometeoroid impacts are a significant and continuing source of particles on orbit.
Outgassing experiments in space were conducted during the critical period in the cryogen lifetime of the large infrared telescope called Spatial Infrared Imager and Telescope (SPIRIT III) on the Midcourse Space Experiment (MSX) spacecraft. This was the period when the solid hydrogen in the dewar was being depleted and the optical components were warming up to evaporate previously condensed volatile materials. The volatile condensable materials were collected on the cryogenically cooled surfaces during the 4 months of prelaunch testing and the 10 months in orbit. The contamination instruments on board the spacecraft were used to monitor the outgassing of these materials. Besides contamination monitoring, it was also desired to control the heating or warm-up process without contaminating the still functioning UV and visible sensors. After considering several scenarios via thermal modeling, it was decided to conduct the warm-up period into two phases, with the first phase intended to approach but not exceed the sublimation point of ice on the primary mirror. Solar radiation was used to heat the SPIRIT III baffle and parts of the +Y face of the spacecraft while the contamination instruments were monitored the outgassing event. Ice redistribution from the baffle to the much colder primary mirror, as well as external pressure bursts and slight film depositions on quartz crystal microbalances were observed. The second phase of warm-up experiments again used solar heating to drive the telescope optics through the 150 K range for final sublimation of any ice remaining as well as condensed hydrocarbons from the cold primary mirror. The results of these end-of-cryo experiments are discussed in terms of the measured film deposits on the cryogenic quartz crystal microbalance and the pressures from the total pressure sensor.
When used on a spacecraft to measure outgassing/erosion rates, a 50 MHz QCM is twenty-five times more sensitive than a 10 MHz QCM, according to theoretical considerations. In a continuation of extending the sensitivity of the QCMs, the frequency range, which is a measurement of mass flux, has increased from 15 MHz to 25 MHz to a now reported 50 MHz crystal sensor. As reported in the previous investigation of the 25 MHz crystal by Wallace, et. al., we again used a thin film interference technique to determine the mass sensitivity of the crystal. Water ice was used as the deposition film and, from known density, interference properties of the resulting film gave the resulting film thickness. Thus the sensitivity of the crystal and also the mass range of operation, with the driving electronics, could be determined. Theoretical sensitivity of the 50 MHz crystal would be 5.657 X 109 Hz/g/cm2 or 0.176 ng/cm2-Hz.
This paper describes a program for consolidating data from quartz crystal microbalances (QCMs) that will enable one to rapidly locate previous measurements on specific materials and data from past space flight experiments. When complete, the databases will contain information on materials outgassing obtained using the ASTM-E-1559 standard, and flight observations of mass accumulations. Once established, these databases will be available to the entire community and will provide a valuable source of material outgassing information. The data should be useful to those working in the Contamination area for mission design and materials specification. Data are being accumulated from both national and international sources. The space flight database will include data from past NASA missions, as well as DOD [including the BMDO-sponsored Mid-course Space Experiment (MSX) program], Canadian Space Agency, European Space Agency, Russian MIR space station, and eventually, the International Space Station. A website is being generated which will be the vehicle for storing the data that are accumulated. Once completed, the databases will be managed by the NASA/Space and Environmental Effects (SEE) Program Office at the Marshall Space Flight Center in Huntsville, Alabama.
Many contamination lessons have been learned since the Midcourse Space Experiment satellite was launched on April 24, 1996. FOremost is the inclusion of spacecraft design and thermal engineers with the contamination team early in the program, which resulted in the placement of vents away from the optical sensors, the separation into clean and 'dirty' sections, the exclusion of thrusters, the use of non- perforated silver/Teflon on the optical sensor axis, and the outgassing procedures instituted for all subsystem suppliers. The use of contamination instruments as monitors during integration and testing enabled correct technical decision to be made during several contamination incidents. In space, the contamination monitoring instruments supported programmatic decisions during the early mission planning. During the warm-up of the cryogenic telescope, it was learned that a small gap between the multilayer insulation and the internal baffles contributed to mass redistribution of water vapor. Consequently, it was also learned that a careful warming procedure may potentially be used to clean space-based cryogenic mirrors of condensed water vapor. Particles brought from the ground can be released by mechanical motions such as door openings as well as by thermal shock induced by the Sun during umbra exit. Solar- induced water evaporation from multilayer insulation can dominate the gaseous environment over the spacecraft's lifetime of several years.
Particle occurrence rates, velocities, size distributions, and trends in the environment have been measured above the Midcourse Space Experiment satellite using optical sensors. Results from the spacecraft's first year on orbit are presented. Particles were detected during relatively quiescent times and as a result of distinct particle release events. On 11 November 1996, we observed a discrete particle release even that is not attributable to spacecraft activity. We hypothesize that this event was caused by an impact by either orbital debris or a micrometeoroid. We present the particle size and velocity distributions from that event and compare them to the quiescent distributions and to previous model predictions.
The Total Pressure Sensor (TPS) on-board the Midcourse Space Experiment (MSX) Spacecraft has continuously measured the ambient local pressure since launch of MSX on April 24, 1996. The primary goals of the sensor are: 1) to monitor the ambient pressure surrounding the spacecraft's optical telescopes and to indicate when environmental conditions are acceptable for opening the protective covers, and 2) to monitor the long-term decay of the species outgassed from the spacecraft. The water-induced environment was expected to rapidly decay over the first few months to elves more closely approaching the natural environment. The data generally shows decay toward this level, however, the pressure is quite variable with time and can be influenced by discrete illumination and spacecraft orbital events. Several experiments, conducted approximately one year into the mission, indicate that the thermal blankets retain significant quantities of water. The local pressure due to water vapor is shown to increase by a factor of 100 from direct solar illumination of the blankets. Moreover, the multi-layer construction of the blankets causes them to form a deep reservoir, which continues to be a source of water vapor several tens of months into the mission. Additionally, the TPS has monitored numerous events in which the measured ambient pressure on the optics deck has exceeded 10-9 Torr. Several of these events did not include solar illumination of the blankets. These events indicate that sources other than the MLI blankets are the cause for certain high-pressure transients. Finally, these events are not limited to the early mission, outgassing phase of the program. They have been witnessed over a year into the mission. The results documented herein indicate that special consideration must be given in the design of optical sensors to account for long term outgassing of a spacecraft.
There is a current need for a sensor which can measure minute outgassing or erosion over very long time spans in the space environment. One way of addressing this need is a QCM with very stable output and high mass sensitivity. In order to increase the mass sensitivity. In order to increase the mass sensitivity of the QCM, the crystal has to oscillate at a higher frequency. In the past, assurance that the mass sensitivity at 10MHz as predicted by theory has been provided by nine different experimenters using the same or different techniques. When 15MHz QCMs with an increased theoretical sensitivity became available, they were experimentally exposed to the same molecular source flow as the 10MHz QCMs, to measure their response. It proved to be identical to theory. Historically, QCM sensor discussions have dealt exclusively with plano-plano crystals, i.e., both sides flat and parallel. Now, however, increases in frequency beyond 15MHz call into question whether we still have plano-plano crystals or whether plano-convex now best describes the crystals. Since the diameter of the high frequency crystal has to be less in order for it to oscillate, it becomes harder and harder to assure true plano-plano crystal performance as the fundamental frequency is raised. In this paper, we will discuss experiments which have been performed comparing the mass sensitivity of 25MHz to 15MHz crystals, or the mass range that is available with these high sensitivity crystals. We will also address the plano-plano versus plano-crystals' sensitivities.
The MSX Contamination Experiment team was responsible for establishing design and operational guidelines and the contamination control plan for the Midcourse Space Experiment (MSX), as well as for tracking hardware cleanliness prior to launch. The approaches taken and the results are described.
This review paper focuses on measurement techniques and facilities for the study of the contamination and space environment effects on optical and thermal radiative surfaces. Laboratory measurements are reviewed and illustrate how cryogenic and relatively warm surfaces can be affected by contaminants, vacuum, and UV. The laboratory data are used to illustrate the important parameters that require consideration when trying to determine these types of effects on future satellite missions. Optical properties of thin contaminants films, BRDF measurements on cryogenic films, quartz crystal microbalance (QCM) measurements, and UV effects on silicone/hydrocarbon films are presented and discussed relative to their applications to satellite systems. The laboratory data are complemented with flight data from the Midcourse Space Experiment (MSX) satellite. Laboratory results were used to interpret MSX spacecraft flight data. The MSX demonstration and validation satellite program was funded by the Ballistic Missile Defense Organization. MSX had UV, visible, and infrared instruments including the Spirit 3 cryogenic telescope and had several contamination instruments for measuring pressure, gas species, water and particulate concentrations, and condensable gas species. Some of the data collected from the flight QCMs are presented.
The Midcourse Space Experiment (MSX) is a Ballistic Missile Defense Organization (BMDO) demonstration and validation
satellite program that has both defense and civilian applications. MSX has UV, visible, and infrared instruments including
the SPIRIT 3 cryogenic telescope. It also has several contamination measuring instruments for measuring pressure, gas
species, water and particulate concentrations and condensable gas species. A cryogenic quartz crystal microbalance (CQCM)
and four temperature controlled microbalances (TQCMs) are part of this suite of contamination measuring instruments. This
paper describes some of the flight QCM data obtained and analyzed to date. The CQCM is located internal to the SPIRIT 3
cryogenic telescope and is mounted adjacent to the primary mirror. Real-time monitoring of contaminant mass deposition on
the primary mirror is provided by the CQCM which is cooled to the same temperature as the mirror -20 K. The four
TQCMs are mounted on the outside of the spacecraft and monitor contaminant deposition on the external surfaces. The
TQCMs operate at -50°C and are positioned strategically to monitor the silicone and organic contaminant flux arriving at
the UV and visible instruments, or coming from specific contaminant sources such as the solar panels. During the first week
of flight operation, all QCMs recorded deposition in the 10-20 ng/cm2-day (1-2 A/day) range. These TQCM deposition rates
have continuously decreased, and after 270 days mission elapsed time (MET), the rates have fallen to values between 0 and
0. 15 A/day depending on TQCM location. Thermogravimetric analyses (TGAs) on the CQCM and TQCMs have provided
valuable insight into the amount and species of contaminants condensed.
This paper discusses the pressure spike phenomenon measured by the Total Pressure Sensor (TPS) on the Midcourse
Space Experiment (MSX) spacecraft as it passes through aurora! regions. The TPS is an inverted magnetron, cold cathode
gauge that has a range of 10'° to i0 Ton and is sampled at a 1 Hertz rate. The operation of the instrument depends uponthe
conversion of neutral molecules to ions and the measurement of the resultant current. The TPS has monitored large (>10 fold
increase) pressure transients in the auroral regions of the Earth, which have very short temporal width (< 3 seconds). It is
shown experimentally that the injection of electrons into the TPS orifice results in a higher apparent pressure measurement,
presumably due to an increase in the ion formation rate. An equivalent positive ion flux, however, does not noticeably effect
the TPS pressure measurement. It is therefore believed that the on-orbit transients seen by the MSX TPS are consistent with
an increased flux of electrons from the auroral regions into the entrance aperture of the gauge. This paper describes ground
experiments to quantify the phenomenon with a flight spare TPS and correlates these results to the measurements made onboard
the MSX spacecraft.
The midcourse space experiment (MSX) satellite was launched into a 903 Km, 99.4-deg orbit April 24, 1996. It carries imaging spectrometers and radiometers that operate in the UV, visible, and infrared spectral ranges. In addition, it carries several contamination measuring instruments that are being used to characterize the contamination environment on, in, and around the satellite. Five are quartz crystal microbalances (QCMs), four of which are temperature- controlled (TQCMs). They are located on various external surfaces of the spacecraft and are operating at minus 40 degrees Celsius to minus 50 degrees Celsius to measure the condensation of silicone and organic molecules. One is a cryogenic quartz crystal microbalance (CQCM) which is located adjacent to the SPIRIT III infrared cryogenic telescope primary mirror. Its temperature followed the mirror which cooled from 28 to 20 K during the first week of operation. All QCMs recorded deposition in the 10 - 20 ng/cm$2)-day (1-2 angstrom/day) range. Thermo-gravimetric analyses on the QCMs provided insight into the amount and species of contaminants condensed. Data from the QCMs and other instruments in the contamination experiment (CE) suite played an important role in determining when it was safe to open covers on some of the optical instruments.
Effects of contaminants on optical surfaces is a continuing concern for space-based systems-- especially those containing cryogenic optical systems. Effects of contaminant films on mirror bidirectional reflectance distribution function (BRDF) were studied in support of the Midcourse Space Experiment (MSX) Satellite that is scheduled for launch in 1994. This study present experimental results of infrared scattering measurements made on cryogenic optical surfaces that are cooled to temperatures as low as 15 K. At this temperature gases such as nitrogen, oxygen, argon, carbon monoxide, carbon dioxide, and water will condense. These are the gases of most concern to the MSX Program. BRDF studies have been completed to determine infrared (10.5 micrometers ) effects as functions of film thickness and angle of reflection for these gases. It has been found that the change in mirror performance depends on contaminant film species, thickness, wavelength, and mirror temperature. Results of the infrared scatter data are compared with previously obtained visible scatter data.
Four temperature-controlled quartz crystal microbalance (QCM) instruments are on the Midcourse Space Experiment (MSX) spacecraft to measure the accretion of contaminants and to identify the contaminant species using the thermogravimetric analysis technique. Calibration data were used to derive equations for the performance of the thermoelectric temperature controllers in the MSX QCM sensors. The equations relate input power and the crystal temperature to the base temperature. The MSX thermal model with the QCM locations was used to predict the operating temperature of each of the radiators. On orbit, the equations can be used to calculate the QCM radiator temperature from the crystal temperature and the input power.
Operation of thermal control and optical surfaces can be impaired by contamination deposition for space-based systems. Determining the contamination level about the spacecraft and deposition of silicones and hydrocarbons will be studied during the flight of the Midcourse Space Experiment Satellite that is scheduled for launch in 1994. The contaminants will be measured using four thermoelectrically cooled quartz crystal microbalances (TQCMs) which will be mounted on external spacecraft surfaces. Rigorous characterization and calibration measurements were made on six flight TQCMs at the Arnold Engineering Development Center at Arnold Air Force Base, TN. Since the TQCM output frequency depends on some degree on both the crystal temperature and the oscillator-mixer temperature, the magnitude of these effects and the effects of solar irradiation on the TQCMs was established. Long-term frequency drift rates were also determined. The change in TQCM output frequency expected to occur in space was, in some cases less than the magnitude of the frequency changes caused by temperature and solar fluctuations. The results of this study will allow a more accurate assessment of the contamination effects that can be expected during long-term space-flight programs.
Contamination of infrared sensor systems is an area of concern to spacecraft operators and designers. This paper describes the CALCRT mathematical model which can be used for calculating the reflectance and transmittance effects of thin contaminant films on optical surfaces. It uses an optical property data base that contains the cryogenic optical properties (refractive and absorptive indices) for satellite material outgassing products, atmospheric gases, propellants, and bipropellant plume products. The bipropellant plume data were obtained during firings of a 5 lb. thrust monomethylhydrazine/nitrogen tetroxide engine. Contaminant effects can be calculated as a function of substrate refractive index, incidence angle, film thickness, film materials, wavelength, and wavenumber. The model has been modified to treat the case when the condensate contains a mixture of molecular species or when a deposit on a surface consists of a `stack' of uniform layers, each of which has a distinct material composition. Computed transmittance spectra of selected films of mixtures are compared with transmittance spectra calculated with measured optical constants of the mixture.
The effect of condensates on optical surfaces is a continuing concern for space-based optical systems such as the Midcourse Space Experiment. Many such systems contain cryogenic optical surfaces that operate on low temperatures where gases such as nitrogen, oxygen, carbon dioxide, and water will condense. This study presents the effects of these gases on mirror surfaces at temperatures as low as 15 K under high vacuum conditions. The bidirectional reflectance distribution function was determined for these condensates in various film thicknesses up to 8 mm. Optical scatter, thickness, and density measurements were obtained simultaneously with the superpolished quartz crystal microbalance (SPQCM). Correlations between thin film deposition, as determined by the SPQCM, and the expected increase in optical scatter are shown. These correlations are important in determining launch decisions in cases where various degrees of condensation may have occurred on cryogenic optical systems during ground processing.
This paper presents the results of an investigation of the operational characteristics of two cryogenically cooled quartz crystal microbalances (CQCM) which are flight units for the Midcourse Space Experiment (MSX) program. The units were operated at temperatures that ranged from 15 K up to 300 K. During the course of this investigation, the CQCMs were temperature cycled over this range for 5 complete warmup/cooldown cycles using warmup rates of 2.5 K/min and 1.0 K/min. There was concern over stop/start operations on orbit wherein the CQCM power could be turned off for some time. The CQCMs were cycled in this manner to determine the return frequency variations after power restoration. A three week drift test at 15 K was carried out to determine the drift in CQCM frequency with time. Temperature effects of the heat sink attached to the CQCM base were also determined by varying this temperature. Finally, films of nitrogen, oxygen, carbon dioxide, and water were deposited at the 15 K base temperature, and a thermogravimetric analysis was done for each of the gases condensed, both for individual gases and for gas mixtures. The results of these analyses will be used to interpret on-orbit analysis of contaminants that are condensed during operation of the SPIRIT III telescope on the MSX satellite.
Many systems contain cryogenic optical systems that operate at temperatures where gases such as nitrogen, oxygen, carbon dioxide, and water will condense. This study presents experimental results of the effects of these gases condensed on highly polished (superpolished) mirror surfaces cooled to temperatures as low as 15 K under vacuum conditions. Using these gases as contaminants, the bidirectional reflectance distribution function was obtained at a wavelength of 0.6328 micron for various contaminant film thicknesses up to 8 microns. Most of the data were obtained using as the mirror surface the superpolished sense crystal of a previously developed quartz crystal microbalance (SPQCM). The SPQCM allowed the mass of the actual contaminant layer to be measured directly.
The bidirectional reflectance distribution function (BRDF) of both white and black, flown and unflown Space Shuttle thermal protection system tiles has been measured in the 2.0-5.5 micron range. The BRDFs of flown and unflown white tiles were nearly identical, and typical of a good diffuse reflector (with nearly constant BRDF as a function of reflectance angle. The BRDF of the black tile was modified as a result of experiencing Space Shuttle flight; spectral BRDF in this case became a monotonic decreasing function of radiation wavelength.
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